Aim To understand: (1) how temperature, precipitation and stand age control the above-ground biomass carbon density (BCDa) of mature forests and its macroecology patterns across latitudes; (2) the age threshold for old-growth forests at a global scale. Location Global forests. Methods We compiled a database (897 sites) of mature forests between 80 and 1200 years old. The site data include latitude, longitude, mean annual temperature, mean annual precipitation, forest type, stand age, BCDa, living biomass (above- and below-ground biomass) carbon density and total (living plus dead) biomass carbon density. Based on the site data, we performed regression analyses to show how BCDa changes with climate and forest stand age. Results At a [...]